1. Field of the Invention
The present invention relates to a method and an apparatus for producing a recording sheet, in particular, for producing a recording sheet having a coloring material receiving layer, which contains inorganic particulates and a water-soluble resin, provided on its substrate, such as a high quality ink jet recording sheet.
2. Description of Related Art
Various types of ink jet methods have been developed up to date; and with their development, various types of ink jet recording sheets have also been developed. Substrates used in these ink jet recording sheets include, for example, not only a variety of resin films but also plain paper, wood free paper (coat paper), and photographic paper. Of these ink jet recording sheets, those provided with a coloring material receiving layer which contains inorganic particulates of fine diameter and a water-soluble resin are now in use as high quality ink jet recording sheets. The recording sheets provided with a coloring material receiving layer containing inorganic particulates and a water-soluble resin also include, for example, those for thermal transfer recording and for electrophotography.
With respect to the production of a recording sheet provided with a coloring material receiving layer containing inorganic particulates and a water-soluble resin, there is disclosed a method in Japanese Patent Laid-Open No. 62-111782 in which a crazing-free recording sheet is produced, while maintaining high ink absorption, high water fastness, and high ink absorption rate, by dividing the coloring material receiving layer into two layers and applying coatings on the layers in sequence. In Japanese Patent Laid-Open No. 8-72388, there is disclosed a method in which a recording sheet of high glossiness is produced by first applying pseudo-boehmite solution to a paper base, second applying silica coating solution to the pseudo-boehmite layer when a water content of the pseudo-boehmite layer is 100 to 450%, and immediately after this, pushing a heated smooth mold against the layer surface. Further, in Japanese Patent Laid-Open No. 11-172597,there is disclosed a method in which a recording sheet of high glossiness is produced by first adjusting a moisture content of its ink coloring material receiving layer formed by applying a coating on a substrate to 200 to 500% by weight, laminating organic polymer films with an arithmetic mean value of surface roughness of 25 nm or less, drying to a moisture content of 5% by weight or lower, followed by stripping the films. In Japanese Patent Laid-Open No. 11-115308, there is disclosed a method in which a recording sheet having high ink absorbability and causing less bleeding and no crazing is produced by providing a coating film of its coloring material receiving layer, which contains inorganic particulates of fine diameter and a water-soluble resin, with a crosslinking agent for crosslinking the water-soluble resin before the above coating film enters its falling drying rate period.
However, in case of the method disclosed in Japanese Patent Laid-Open No. 62-111782, when applying a coating for forming a top layer, such as protective layer, on a sublayer formed by applying/drying a coating for a coloring material receiving layer containing fine particles, bubble trouble is likely to occur, as described in Japanese Patent Laid-Open No. 9-156202, which is a phenomenon that air trapped in voids created in the coloring material receiving layer due to the existence of particulates appears on the protective layer surface of the top layer in the form of traces of bubbles. As a result, the conditions of the coated surface of the recording sheet, as a product, deteriorate, and the smoothness of the same is decreased. Thus the method has a disadvantage in that it is impossible to obtain a recording sheet of high glossiness.
In cases of the methods disclosed in Japanese Patent Laid-Open No. 8-72388 and in Japanese Patent Laid-Open No. 11-172597, the coated surface of a recording sheet is pushed with a mold or laminated with organic polymer films so as to be smoothened; accordingly drying the coating film must be conducted from the back side (the substrate side) of the recording sheet. Thus these methods have disadvantage in that, when using an impermeable film for the substrate of a recording sheet, it is impossible to proceed with drying the coating film.
In case of the method disclosed in Japanese Patent Laid-Open No. 11-115308, in the process of providing a crosslinking agent to an ink coloring material receiving layer, the crosslinking agent is applied on the coloring material receiving layer which does not enter its falling drying rate period yet and is still in the wet state. Accordingly, the conditions of the coated surface of a recording sheet, as a product, may deteriorate depending on the composition and physical properties of the coating solution and on the conditions of coating operation, in particular, the properties of the crosslinking agent, the coating speed, and the coated width; the type of the substrate; and the properties and surface conditions of the coloring material receiving layer containing inorganic particulates and a water-soluble resin in itself. This causes defects in quality, in addition, influences the stability of production because the production is under restrictions of the composition and physical properties of the coating solution as described above. In the cases where a crosslinking agent is applied on the coloring material receiving layer using a slide coater, an extrusion coater or a curtain coater via a bead or curtain film, the bead portion and the curtain film are affected by the entrained wind occurring due to the travel of the substrate and is likely to vibrate; thus the coating of the crosslinking agent is likely to be non-uniform in a direction in which the substrate travels or in a transverse direction of the substrate. In addition, the application of a crosslinking agent is likely to be affected by the wettability by the crosslinking agent on the coloring material receiving layer and affected by the progress of the crosslinking reaction. And when the application of the crosslinking agent is thus destabilized, lines and unevenness are likely to occur on the surface of the products having been subjected to drying. The occurrence of such lines and unevenness not only causes the appearance of the products to deteriorate, but also causes deficiencies in performance, such as glossiness and image quality, of the products as a recording sheet. Particularly in terms of reactivity of the coloring material receiving layer with a crosslinking agent, it is necessary to cure the coloring material receiving layer early by the crosslinking agent, and the reaction is already in progress when a bead is formed; thus the stability in the application of the crosslinking agent is largely affected by the progress of the crosslinking reaction. When using coaters other than those described above, such as roll coater and bar coater, the meniscus of the crosslinking agent solution formed between the roll or bar and the coloring material receiving layer still in the wet state is disturbed depending on the conditions such as wettability by the crosslinking agent on the coloring material receiving layer, reactivity of the coloring material receiving layer with the crosslinking agent and coating speed of the crosslinking agent, which is likely to cause lines and unevenness on the surface of the products having been subjected to drying. When using a dip coater, the amount of the crosslinking agent to be coated cannot be freely controlled. This causes problems in that the formulation and the performance of the crosslinking agent are largely affected, and a worse condition, an excess amount of crosslinking agent applied sags and runs, causing lines on the surface of the products. Further, when using a spray coater for applying a crosslinking agent to the coloring material receiving layer, the traces of the droplets of the crosslinking agent may remain on the products having been subjected to drying, depending on the size of the droplets, the wettability, the coating speed and the coating amount of the crosslinking agent sprayed, causing speck-like lines and evenness thereon.
Then, the present inventors attempted to use a casting drum as measures to resolve these disadvantages. Specifically, after applying a coating on a sublayer to form a top layer or after applying a crosslinking agent on a coloring material receiving layer in the wet state, the present inventors attempted to resolve a decrease in smoothness, lines and unevenness occurring on the coated surface by bringing a casting drum having been subjected to mirror finish into contact with the coated surface and applying temperature and pressure thereto. However, in order to resolve the disadvantages, a casting drum with high temperature and high pressure is needed, and the present inventors found that a currently used casting drum equipment was insufficient to do such a thing. In addition, in the cases where a substrate is impermeable, like a film, a casting drum cannot be used because the progress of drying is not expected. Thus the present inventors"" attempt was found to be not a thorough solution to resolve the disadvantages.
On the other hand, unless the above disadvantages are resolved, the quality of the products, such as glossiness, cannot be improved and moreover the yields of the products are decreased due to the existence of lines and unevenness, and the productivity is thus reduced.
And in order to produce a product of high glossiness in a good yield in a stable manner by the currently used methods as described above, it is necessary to optimize not only the coating methods and the conditions of coating operation used in applying the crosslinking agent, but also the physical properties of the crosslinking agent and the coloring material receiving layer. This means that the conditions of a manufacturing process such as the conditions of coating operation and the design of the composition and physical properties of the coating solution are under many restrictions. As a result, great influences on the currently used methods are produced such that the stable manufacturing conditions are within narrow limits. Further, even if a casting drum is installed which can be used at a high temperature and a high pressure, a heavy investment in plant and equipment is required for remolding and optimizing the apparatus and moreover the casting drum is not applicable to the products using a film as a substrate.
The present invention has been made in light of these situations. Accordingly, an object of the present invention is to provide a method and an apparatus for producing a recording sheet provided with a coloring material receiving layer containing inorganic particulates and a water-soluble resin, thereby the production of such a recording sheet can be free from restrictions of the composition and properties of a coating solution as well as restrictions of the conditions of coating operation, the coated surface of the recording sheet produced is of high glossiness, and the production of such a recording sheet can be carried out in a stable manner without causing lines and unevenness on the coated surface of the recording sheet produced.
In order to accomplish the above object, the present invention is directed to a method of producing a recording sheet that includes the steps of applying a coating on a substrate to form a coloring material receiving layer containing inorganic particulates and a water-soluble resin, applying at least one coating on the coloring material receiving layer to form at least one top layer, and performing, immediately after the topmost layer of the coatings is formed, smoothing/metering treatment to a surface coated with the coloring material receiving layer and the at least one top layer, wherein the substrate is one of plain paper, coat paper, photographic paper, and plastic.
Further, in order to accomplish the above object, the present invention is directed to an apparatus for producing a recording sheet, comprising: a coloring material receiving layer coater which applies a coating on a substrate to form a coloring material receiving layer containing inorganic particulates and a water-soluble resin; a first dryer which dries the coloring material receiving layer; a top layer coater which applies, after or during drying the coloring material receiving layer with the first dryer, at least one coating on the coloring material receiving layer to form at least one top layer; a smoothing/metering unit which performs smoothing/metering treatment to a surface coated with the coloring material receiving layer and the at least one top layer, the smoothing/metering unit being provided to a downstream side of the top layer coater; and a second dryer which dries the at least one top layer having been performed with the smoothing/metering treatment.
According to the present invention, the coated surface of a recording sheet is subjected to smoothing/metering treatment immediately after the topmost layer of multiple coatings is formed. This makes it possible to resolve the deterioration of the coated surface conditions caused by bubble trouble, which is likely to occur when top layers are formed by applying multiple coats on the color material receiving layer containing inorganic particulates and a water-soluble resin, as well as the lines and unevenness occurring on the coated surface due to the unstable application of a coating; accordingly a recording sheet having a coated surface of satisfactory conditions and of high glossiness can be produced. On top of that, in the method of the present invention, since unfavorable conditions occurring on the coated surface, such as bubble trouble, lines and unevenness, are repaired after the formation of the coated surface, the method is free from restrictions of the composition and physical properties of the coloring material receiving layer and the top layers, of the conditions of coating operation, and of the types of the coater used; thus a recording sheet having a satisfactory coated surface can be produced in a stable manner. With respect to the smoothing/metering units, those having a simple structure, such as bar type-, air knife type- and blade type- smoothing/metering units, can be used; accordingly a large-scale remolding is not required for the currently used apparatus for producing a recording sheet.